1. Field of the Invention
Aspects of the present invention relate to a zeolite adsorbent for desulfurization, and more particularly, to a zeolite adsorbent for desulfurization that is excellent for removing sulfur compounds, such as tetrahydrothiophene (THT) at ambient temperature.
2. Description of the Related Art
A fuel cell is a generation system that directly converts chemical energy of hydrogen and oxygen contained in hydrocarbon-based material, such as methanol, ethanol and natural gas, to electrical energy.
Such a fuel cell is typically provided with a stack, a fuel processor (FP), a fuel tank, a fuel pump. The stack makes up the main body of a fuel cell, and has a structure in which several or dozens of unit cells, each made up of a membrane electrode assembly (MEA) and a separator (or bipolar plate), are stacked. The fuel pump feeds a fuel from a fuel tank into a fuel processor, and the fuel processor reforms and purifies the fuel to generate hydrogen and feeds the hydrogen to the stack. The stack receives the hydrogen and electro-chemically reacts the hydrogen with oxygen to generate electrical energy.
A reformer and a water-gas shift reactor in the fuel processor reform hydrocarbons by employing catalysts. The hydrocarbons to be reformed typically contain sulfur compounds, and the above-mentioned catalysts, as well as an anode catalyst of a membrane electrode assembly, are easily poisoned by sulfur compounds. Accordingly, it is required that the sulfur compounds be removed before feeding the hydrocarbons to the reforming process. Thus, the hydrocarbons should go through a desulfurization process before entering the reforming process (see FIG. 1).
Particularly, city gas (i.e., gas from a municipal utility) may be used as a feedstock of fuel cells. City gas typically contains about a few ppm of sulfur compounds that are added to the gas to function as odorants. Typically, the sulfur compounds added to city gas are tertiary butyl mercaptan (TBM) and tetrahydrothiophene (THT). As discussed above, it is necessary to remove the sulfur compounds in order to use the city gas in fuel cells.
A method of removing sulfur compounds can employ a hydrodesulfurization (HDS) process or an adsorbent. Although the hydrodesulfurization process is reliable, the process requires high temperatures of 300° C. to 400° C., is complicated to operate and thus is more suitable to large scale plants than to small scale devices.
To remove sulfur compounds such as TBM, THT, etc., in a fuel gas, a method employing an adsorbent is more suitable. The method employing an adsorbent passes a fuel gas through an adsorbent made of activated carbon, metal oxide or zeolite, etc., to remove the sulfur compounds. When the adsorbent becomes saturated with sulfur compounds, so that it is no longer able to remove additional sulfur compounds, replacement or regeneration of the adsorbent is required. The amount of needed adsorbent and the replacing cycle of the adsorbent are largely influenced by the adsorptivity of the adsorbent, and thus an adsorbent having high adsorptivity is preferred.
Various adsorbents have been proposed. JP 1994-306377 B discloses a zeolite ion-exchanged with multivalent metal ions to remove mercaptans contained in city gases. However, the zeolite could be applied to mercaptans only.
Further, it is known that THT is more difficult to remove than TBM. A zeolite containing Ag has the ability to remove THT.
JP 10-237473 B discloses a Na-X-zeolite in which the pore size is at least 5 Å, as an adsorbent. The adsorbent shows excellent adsorptivity at ambient temperature; however, when it is exposed to moisture the adsorptivity is drastically reduced.
US 2004-57890 A discloses an adsorbent that shows relatively excellent adsorptive performance at ambient temperature even when exposed to moisture. The adsorbent is obtained by ion-exchanging X zeolite, Y zeolite or beta zeolite with transition metal elements, such as silver, copper, etc.
However, for the zeolite adsorbents disclosed up to the present date, the crystallinity of the zeolites has not been emphasized, and insufficient attention has been paid to the relationship between crystallinity and improved performance of zeolite adsorbents.